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Non-native freshwater plants

New Zealand pygmyweed

New Zealand pygmyweed (Crassula helmsii) is also known as Australian swamp stonecrop and erroneously as Tillaea helmsii or Tillaea recurva. It is an invasive, non-native, aquatic plant. The appearance of this species is variable, but is recognisable by its fleshy, light-green leaves and small, white flowers. The plant is described more fully in this identification guide  PDF document .

History of invasion and expansion

New Zealand pygmyweed is a native species of Australia, New Zealand and Tasmania. It was first introduced for sale in England in 1927, but until 1956 it was recorded only in water bodies that had been planted-up. Since the late 1970s, it has spread rapidly north and west. Between 1970 and 1986, it was recorded in 33 10 km squares in Britain. In contrast, between 1987 and 1999, it was recorded in 574 10 km squares in Britain and Ireland. It is now widespread and locally abundant in England.

There are records of New Zealand pygmyweed at more than 50 sites in Scotland, but it is believed that the species is under-recorded. In general, the majority of records have been made in lowland, coastal and central locations. The sites in which the species has been found include ponds that were planted-up, some of which are associated with Sustainable Urban Drainage Systems (SUDS). It appears likely that this species was introduced into Scotland as a result of its availability from plant retailers, either directly through sale of this plant, or because of contamination of other plants and soil with viable fragments. Once introduced, the species can be spread by users of the affected water body. Further information on the distribution of New Zealand pygmyweed in Scotland is available in this report.

General ecology

New Zealand pygmyweed grows submerged in sheltered waters up to 3 m deep, but it emerges above the water's surface in shallow water. This species also colonises damp ground. It grows on soft substrates in a variety of habitats, including ponds, lakes, reservoirs, canals and ditches, and is tolerant of a range of conditions of water chemistry.

New Zealand pygmyweed produces flowers on emergent stems, with separate male and female flowers developing on the same plant. It is not known whether the seeds produced in Britain germinate, but spread occurs vegetatively through growth of small fragments of the plant. Fragments may be produced by disturbance or damage to plants, but small shoots may also detach from the parent plants during autumn. Plants do not die back in winter and live for a number of years.

Impacts on other biodiversity and conservation interests

New Zealand pygmyweed generally grows vigorously in sites to which it is introduced and can form dense, virtually pure stands of vegetation. The resulting high plant biomass may cause detrimental effects to aquatic ecosystems, through limitation of light, depletion of oxygen, changes in pH, or a combination of these factors. Not only can New Zealand pygmyweed outcompete native plants, but its dominance may suppress biota that normally use native plants as a substrate, or source of food or shelter.